Gaskets

ABSTRACT

A gasket comprises a sealing layer formed from a resilient material which comprises particles of chemically-exfoliated vermiculite bonded together. The layer also comprises a hydrolysis-resistant polymer coupled to the vermiculite by a coupling agent. A further gasket comprises a sealing strip wound into a spiral. The sealing strip comprises a resilient layer comprising particles of chemically-exfoliated vermiculite. The sealing strip also comprises a flexible carrier strip to which said resilient layer is bonded.

The invention is concerned with gaskets (including spirally woundgaskets), in particular with gaskets having a sealing-enhancing layerwhich is based on exfoliated vermiculite.

Exfoliated vermiculite is a known heat-resistant resilient material.Exfoliated vermiculite is conventionally formed by expanding mineralvermiculite using gas, this material being referred to herein as"gas-exfoliated vermiculite". The gas may be thermally generated, inwhich case the product is called "thermally-exfoliated vermiculite"(TEV). TEV may be made by flash-heating mineral vermiculite to 750-1000°C., at which temperature the water (free and combined) in the orevaporises rapidly and the steam generated forces apart the silicatesheets which form the raw material, so bringing about an expansion of10-20 times perpendicular to the plane of the sheets. The granulesformed have a chemical composition which (apart from the loss of water)is virtually identical to that of the raw material. Gas-exfoliatedvermiculite may also be made by treating raw vermiculite with a liquidchemical, eg hydrogen peroxide, that penetrates between the silicatesheets and subsequently evolves a gas, eg oxygen, to bring aboutexfoliation. A different form of exfoliated vermiculite is known as"chemically-exfoliated vermiculite" (CEV) and is formed by treating theore and swelling it in water. In one possible preparation method, theore is treated with saturated sodium chloride solution to exchangemagnesium ions for sodium ions, and then with n-butyl ammonium chlorideto replace sodium ions with n--C₄ --H₉ NH₃ ions. On washing with waterswelling takes place. The swollen material is then subjected to highshear to produce an aqueous suspension of very fine (diameter below 50microns) vermiculite particles.

It is known to utilise exfoliated vermiculite as a layer of a sheetgasket, eg an automotive head gasket, and for other purposes. Forexample, GB 2 193 953 B discloses forming sheet-like gaskets formed fromparticles of gas-exfoliated vermiculite. Because such particles do notcohere well, they are bound together by fine particles of CEV. The useof CEV as a binder retains heat resistance and resilience, whereas theuse of other binders could result in an incompressible structure.However, although exfoliated vermiculite has excellent heat resistanceand a high degree of resilience, it has poor water resistance.

GB 2 123 034 B describes making a flexible sheet material, eg for agasket, by subjecting an aqueous suspension to electrophoresis. Thesuspension contains an expanded layer silicate, eg CEV with a particlesize below 50 microns, and a dispersed organic polymeric material, egacrylic polymer, acrylonitrile-butadiene copolymer, epoxy resin, ornatural rubber.

A sealing element for a gasket for the exhaust system of an internalcombustion engine is disclosed in GB 2 217 742 A. This sealing elementcomprises relatively coarse particles of TEV (passing a 2 mm sieve)bonded together by fine CEV particles (about 100 microns in size). Thiselement is stated to disintegrate quickly if exposed to water as thefine CEV particles are readily dispersed in water. In order to improvewater-resistance, GB 2 217 742 A proposes bringing the element intocontact with a solution of an aluminate or a zirconyl salt. Furtherimprovement is achieved by treatment with a solution of a siliconeelastomer. An example is given of impregnation of a sheet (which hadalready been treated with sodium aluminate) by a 15% solution ofsilicone elastomer in toluene, the solids uptake being 3% by weight.

It is an object of the present invention to provide a gasket comprisinga layer of sealing enhancing material which is based on exfoliatedvermiculite, the layer having improved water resistance.

The invention provides a gasket comprising a sealing layer formed from aresilient material which comprises particles of chemically-exfoliatedvermiculite bonded together, wherein the layer also comprises ahydrolysis-resistant polymer coupled to the vermiculite by a couplingagent.

In a gasket according to the invention, it is found that the layer ismore water resistant than a material containing only vermiculite and acoupling agent, and also more water resistant than a material containingonly vermiculite and a polymer.

Since CEV is a relatively expensive material compared withgas-exfoliated vermiculite, eg TEV, in a gasket according to theinvention, the resilient layer may also comprise particles ofgas-exfoliated vermiculite, eg the layer may comprise particles ofgas-exfoliated vermiculite bonded together by particles of CEV. Thegas-exfoliated vermiculite may be milled to a particle size of less than50 microns. Other possible additives include talc, mica and unexfoliatedvermiculite.

The polymer may be selected from nitrile butadiene rubbers, styrenebutadiene rubbers, natural rubber, butyl rubber, siloxanes (particularlyorganosiloxanes such as dialkyl siloxanes) and ethylene propyldienemonomer. Diene-based polymers are suitable because they are flexible andhyrolysis-resistant.

The coupling agent may be a silane, eg a vinyl functional silane such astriethoxy vinyl silane (CH₃ CH₂ O)₃ SiCH═CH₂).

Said resilient layer may be mechanically bonded to a sheet of thegasket, eg by tangs projecting from the sheet into the layer. The sheetmay be of stainless steel, carbon steel, wire mesh or fibre mesh, suchas glass fibre mesh.

Spirally wound gaskets are well-known and are formed from a metalsupporting strip, conventionally of steel, and a sealing strip formedfrom a resilient material, conventionally expanded graphite (also calledexfoliated graphite). In the formation of conventional spirally woundgaskets, the steel supporting strip is fed onto a mandrel. The steelsupporting strip is welded either to itself to form a closed loop aroundthe mandrel or, alternatively, is welded to an inner ring of the gasketwhich is itself mounted on the mandrel. The mandrel is then rotated todraw further supporting strip on to the mandrel to form a planar spiral.Simultaneously, the sealing strip is drawn between the coils of thesteel strip so that a spiral of the sealing strip is formed interposedbetween the coils of the supporting strip. When the gasket spiral hasbeen completed, the steel supporting strip is welded to itself to form aclosed loop at the outside of the gasket and the gasket is removed fromthe mandrel. Such gaskets are utilised, for example, for forming sealsbetween flanges at the ends of pipes. The supporting strip holds thesealing strip in position and the sealing strip forms a seal between theflanges and between the coils of the supporting strip.

It should be clear, from the above description of how spirally woundgaskets are formed that, the sealing strip thereof must have sufficientstrength to enable it to be drawn into the spiral without breakage. Asealing strip formed from expanded graphite foil, although relativelybrittle, does have sufficient strength.

In many cases, it is desirable for a spirally wound gasket to have ahigh degree of heat resistance but, in a conventional gasket, the heatresistance is limited by that of the expanded graphite which is lowerthan is desirable.

As discussed above, although exfoliated vermiculite has excellent heatresistance and a high degree of resilience, strips formed fromexfoliated vermiculite bound with CEV are not suitable for use inspirally wound gaskets because such strips are inherently too brittle toallow formation of the gasket, by the method described above, withoutserious risk of breakage of the strip.

It is a further object of the present invention to provide a spirallywound gasket in which the sealing strip has increased heat resistance.

According to a further aspect, the invention provides a gasketcomprising a sealing strip wound into a spiral, wherein the sealingstrip comprises a resilient layer comprising particles ofchemically-exfoliated vermiculite, and a flexible carrier strip to whichsaid layer is bonded.

In a gasket according to the invention, the resilient is bonded to thecarrier strip, so that, during winding of the gasket, the strength ofthe strip prevents breakage of the resilient material. This enables agasket with increased heat resistance to be formed.

The resilient layer may also comprise particles of gas-exfoliatedvermiculite, eg the layer may comprise particles of gas-exfoliatedvermiculite bonded together by particles of CEV. The particles ofgas-exfoliated vermiculite may be milled to a diameter of 50 microns orless. It is also possible for the resilient layer to compriseunexfoliated (intumescent) vermiculite which can, on heating of thegasket, eg in situ, form TEV to swell the resilient layer and, thusimprove sealing.

In order to improve the water-resistance of the gasket, the resilientlayer may also comprise a hydrolysis-resistant polymer coupled to thevermiculite. Suitable polymers are nitrile butadiene rubbers, styrenebutadiene rubbers, natural rubber, butyl rubber, and ethylenepropyldiene monomer. Diene-based polymers are suitable because they areflexible and hydrolysis-resistant. Suitable agents for coupling thepolymer to the vermiculite are silanes, eg vinyl functional silanes,such as triethoxy vinyl silane (CH₃ CH₂ O)₃ SiCH═CH₂).

Said resilient layer and a further resilient layer may be bonded toopposite sides of the carrier strip. This improves sealing by providingseals on both sides of the carrier strip.

The resilient layer may be bonded to the carrier strip by adhesive butit may be advantageous if it is mechanically bonded.

The carrier strip may be made of fabric, paper, glass tissue or plasticsmaterial but, for high temperature applications, it is preferred if itis made of metal. Where the gasket also comprises a separate supportingstrip so that the carrier strip functions only to enable formation ofthe gasket without breakage of the resilient layer, the carrier stripis, preferably, a thin metal foil, eg of aluminium, nickel or steel.However, it is also possible for the carrier strip to function also asthe supporting strip of the gasket, being made of, eg, stainless steel.The resilient layer may be mechanically bonded to a metal carrier stripby tangs projecting from the carrier strip into the resilient layer. Forexample, a strip of tanged metal can be brought into overlyingrelationship with a layer of the resilient material and passed betweenrollers to press the tangs into the resilient material.

Preferably, a metal carrier strip has end portions which are not bondedto the resilient layer so that these end portions can be welded in theformation of the gasket.

There now follows a detailed description of illustrative examplesaccording to the invention.

A tanged stainless steel sheet was first prepared. This sheet was 100microns in thickness. The sheet was tanged by perforating it with squareholes, each hole being 1.5 mm square and the hole centre-spacing being 3mm. Half the holes were perforated by passing a tool through the sheetin a first direction and the remaining half, which alternated with thefirst-mentioned half, were perforated by passing a tool through thesheet in the opposite direction. The edges of the holes, thus, formedtangs projecting from the sheet in opposite directions. The tangsprojected by about 1 mm.

In illustrative example 1, an aqueous slurry (15% solids) was obtainedcontaining about 0.741 Kg of CEV particles (the slurry was obtained fromGrace Construction Products Limited and is designated "Microlite HTS").

The slurry was approximately 15% solids. To this slurry was added 0.074Kg of particles of spray-dried CEV having particle size about 45 micronsobtained from Grace Construction Products Limited and designated"Microlite Powder". To this, was added 0.185 Kg of Dupre Superfine TEV.This gave a paste having approximately 37% solids. To this paste wasadded 3.7 g of a coupling agent (a vinyl functional silane called"Silquest A-151" obtainable from OSi Specialities) and further mixingwas carried out.

Next, a hydrolyses-resistant polymer/solvent mixture was prepared. Thismixture was 50 g of solid nitrile butadiene rubber (Nippon Zeon N36C80),250 g of toluene, and 3.1 g of a curing agent ("Dicup 40",dicumylperoxide). 111 g of this mixture (ie 18.5 g of rubber) was addedto the above-mentioned paste and mixing was carried out. This gave apaste with approximately 5% rubber content.

Next, the paste (including the polymer/solvent mixture) was spread overone side of the metal sheet mentioned above. The sheet was then passedbetween calendering rollers (using release paper to prevent the pastesticking to the rollers) and was dried. Further paste was then spreadover the other side of the metal sheet and the calendering and dryingwas repeated. The sheet was then pressed to densify the resilientmaterial which formed layers approximately 0.75 mm thick on both sidesof the metal. Then it was heated to peroxide cure the rubber.

The completed gasket had two sealing layers formed from a resilientmaterial. The resilient material comprised particles of CEV bondedtogether, and coupled to the nitrile butadiene rubber by the silane. Thegasket was tested to determine its water resistance by boiling in waterfor 5 hours. The gasket retained its integrity.

In illustrative example 2, an aqueous slurry (15% solids) was obtainedcontaining about 0.471 Kg of CEV particles (the slurry was obtained fromGrace Construction Products Limited and is designated "Microlite HTS").The slurry was approximately 15% solids. To this slurry was added 0.529Kg of particles of spray-dried CEV having particle size about 45 micronsobtained from Grave Construction Products Limited and designated"Microlite Powder". This gave a paste having approximately 60% solids.To this paste was added 6 g of a coupling agent (a vinyl functionalsilane called "Silquest A-151" obtainable from OSi Specialities) andfurther mixing was carried out.

Next, a rubber/solvent mixture was prepared. This mixture was 50 g ofsolid nitrile butadiene rubber (Nippon Zeon N36C80), 250 g of toluene,and 3.1 g of a curing agent ("Dicup 40", dicumylperoxide). 90.9g of thismixture was added to the above-mentioned paste and mixing was carriedout. This gave a paste with approximately 2.5% rubber content.

Next, the paste (including the rubber/solvent mixture) was spread overone side of the metal sheet mentioned above. The sheet was then passedbetween calendering rollers (using release paper to prevent the pastefrom sticking to the rollers) and was dried. Further paste was thenspread over the other side of the metal sheet and the calendering anddrying was repeated. The sheet was then pressed to densify the resilientmaterial which formed layers approximately 1.4 mm thick on both sides ofthe metal. It was then heated to peroxide cure the rubber.

The metal sheet was then slit into strips 7 mm wide on a conventionalslitting machine and these strips, thereby forming a metal carrier stripwith resilient layers bonded to both sides thereof. The strips werewound into a spiral gasket by a conventional winding machine. Thecompleted gasket had a spiral of stainless steel strip, acting as asupporting strip of the gasket, with two resilient layers betweenadjacent coils of the steel.

The gasket made according to the illustrative method was heated to 450°C. and held at that temperature for 8 hours. After returning to ambienttemperature, the gasket was subjected to a standard pressure test and noleakage was observed.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

What is claimed is:
 1. A gasket comprising:a sealing strip having a resilient layer with a plurality of chemically-exfoliated vermiculite particles; and a flexible carrier strip, said sealing strip bonded to said flexible carrier strip, said sealing strip and said flexible carrier strip oriented in a spiral relation.
 2. The gasket of claim 1, wherein said resilient layer further comprises a plurality of gas-exfoliated vermiculite particles.
 3. The gasket of claim 1, wherein said resilient layer further comprises a hydrolysis-resistant polymer coupled to said resilient layer.
 4. The gasket of claim 2, wherein said resilient layer further comprises a hydrolysis-resistant polymer coupled to sail resilient layer.
 5. The gasket of claim 4, wherein said hydrolysis-resistant polymer is selected from the group consisting of nitrile butadiene rubbers, styrene butadiene rubbers, natural rubber, butyl rubber, siloxane, ethylene, and propyldiene monomer.
 6. The gasket of claim 3 wherein said polymer is coupled to said resilient layer by a silane.
 7. The gasket of claim 4 wherein said polymer is coupled to said resilient layer by a silane.
 8. The gasket of claim 1 further comprising a second sealing strip having a resilient layer without a plurality of chemically-exfoliated vermiculite particles and opposingly bonded with respect to said first sealing strip to said carrier strip.
 9. The gasket of claim 8, wherein said resilient layer of each of said first and said second sealing strips further comprises a plurality of gas-exfoliated vermiculite particles.
 10. The gasket of claim 8, wherein said resilient layer of each of said first and said second sealing strip further comprises a hydrolysis-resistant polymer coupled to said resilient layer.
 11. The gasket of claim 9, wherein said resilient layer of each of said first and said second sealing strip further comprises a hydrolysis-resistant polymer coupled to said resilient layer.
 12. The gasket of claim 11, wherein said hydrolysis-resistant polymer is selected from the group consisting of nitrile butadiene rubbers, styrene butadiene rubbers, natural rubber, butyl rubber, siloxane, ethylene, and propyldiene monomer.
 13. The gasket of claim 10 wherein said polymer is coupled to said resilient layer by a silane.
 14. The gasket of claim wherein said polymer is coupled to said resilient layer by a silane. 